THIRD ANNUAL YEAR BOOK — PART I. 23 



great gaps along their sides or to carry the surface material away. As 

 the grade increases in steepness either the load has to be diminished in 

 proportion or additional horsepower used. 



Accurate tests have shown that a horse which can pull on a level 

 road 1,000 pounds, on a rise of — 



1 foot in 100 feet can draw only 900 pounds. 



1 foot in 50 feet can draw only 810 pounds. 



1 foot in 44 feet can draw only 750 pounds. 



1 foot in 40 feet can draw only 720 pounds. 



1 foot in 30 feet can draw only 640 pounds. 



1 foot in 25 feet can draw only 540 pounds. 



1 foot in 24 feet can draw only 500 pounds. 



1 foot in 20 feet can draw only 400 pounds. 



1 foot in 10 feet can draw only 250 pounds. 



It will therefore be observed that when the grades are 1 foot in 44 

 feet a horse can draw only three-fourths as much as he can on a level. 

 Where the grade is 1 foot in 24 he can draw one-half as much, and on a 

 grade of 1 foot in 10 he is able to draw only one-fourth as much as on 

 a level road. The difficulty as well as the cost of hauling is therefore 

 necessarily increased in proportion to the roughness of the surface or 

 steepness of the grade. 



LIMIT OP GRADE ALLOWABLE. 



The proper grade for any particular road must be determined by the 

 conditions and requirements existing on that road. The ideal grade is, 

 of course, a level, but as the level road can seldom be obtained in rolling 

 countries, it is well to know the steepest allowable grades for ordinary 

 travel. 



It has been found by experiment that a horse can, for a short time, 

 double his usual exertion. From the above table we find that a horse can 

 draw only about one-half as much on a 4 per cent grade as he can on a 

 level road. As he can double his exertion for a short time, he can pull 

 twice as much more and the slope or grade which would force him to 

 draw that proportion would therefore be a 4 per cent grade. On this 

 slope, however, he would be compelled to double his ordinary exertion to 

 draw a full load, and this will therefore be the maximum grade if full 

 loads are to be hauled. Most road builders prefer 3 per cent grades to 

 those of 4 per cent where they can be secured without additional ex- 

 pense, but in some places it is necessary, for various reasons, to increase 

 the grades to 5 per cent. With the exception of mountainous regions, 

 where steeper grades are often unavoidable, the aim should be, on all 

 public highways which are traveled by heavily loaded vehicles, to keep 

 the grade down to 3 or 4 per cent and never to let it exceed 5 per cent. 



QUESTIONS OF MATERIAL AND EXPOSURE. 



If the road must be constructed out of the materials over which it 

 passes, it is often possible to select a route where the soil is better 



